There are known fuel assemblies for nuclear reactors which are cooled by water, and in particular for nuclear reactors which are cooled by pressurized water and which comprise a cluster of fuel rods which are parallel with each other and which are maintained in a framework which is closed at its ends by end-pieces having a transverse direction relative to the axial longitudinal direction of the fuel assembly.
In the case of pressurized water nuclear reactors, the fuel assemblies are generally of parallelepipedal form with a square base of great length, in the axial direction, for example, 4 meters or more, having a cross-section whose side is approximately 0.20 m long. The end-pieces of the fuel assembly are of square form, and one of the end-pieces located at a first end of the fuel assembly which is in the lower portion of the fuel assembly in the reactor during operation constitutes the bottom end-piece and comprises, at each of its corners, a foot which is intended to move into abutment against a core support plate or lower core plate. Two of the feet located along a diagonal of the bottom end-piece are perforated by holes that are intended each to receive a centering pin of the fuel assembly that projects in the vertical direction upwards on the lower core plate. When the fuel assembly is positioned on the lower core plate, the two holes of the bottom end-piece are engaged on two pins of the lower core plate that are intended to bring about the positioning of the fuel assembly perpendicularly relative to holes for the passage of water extending through the lower core plate. The end-piece that is fixed to the second end of the fuel assembly and that is intended to be located in the upper portion when the fuel assembly is positioned in the nuclear reactor core, and that is referred to as the top end-piece, comprises in particular supporting leaf springs at the upper surface thereof and two centering holes at two corners of the square cross-section of the top end-piece. The centering holes of the top end-piece of the fuel assembly are provided in order to receive centering pins projecting under an upper core plate that is intended to rest on the upper portion of the fuel assemblies of the core comprising the supporting leaf springs. The upper plate of the nuclear reactor constitutes the lower portion of the upper internal pieces of equipment of the nuclear reactor which are positioned above the core, after the fuel assemblies have been introduced into the chamber of the nuclear reactor, so as to bring about the maintenance of the fuel assemblies of the core. When the upper core plate is positioned above the fuel assemblies which are arranged in the core of the reactor, the centering pins of the upper plate of the core which are arranged in the region of each of the fuel assemblies are engaged in the centering holes of the top end-piece of the fuel assembly.
The centering pins of the lower core plate and the upper core plate have an engagement portion of frustoconical form and a cylindrical member whose diameter is slightly smaller than the diameter of the centering hole. The centering pins therefore bring about maintenance of the fuel assemblies in the transverse directions perpendicular to the axial longitudinal direction and in the longitudinal direction, whilst at the same time allowing longitudinal displacements in order to absorb differential expansions in the reactor during operation, the fuel assemblies being maintained in the axial direction by the leaf springs which are able to become deformed by flexion. In the reactor during operation, the pressurized cooling water of the nuclear reactor passes through the fuel assemblies in the axial longitudinal direction thereof and consequently subjects them to a hydraulic thrust in the vertical direction upwards which is manifested by loads which are applied to the structure of the fuel assembly and a slight displacement of the fuel assemblies in the axial direction and lateral vibrations. In order to compensate for the hydraulic thrust and to prevent excessively large displacement of the fuel assembly under the action of the hydraulic thrust, it is necessary to provide leaf springs that have powerful characteristics. Such leaf springs apply great compression loads to the structure of the fuel assembly so that it is necessary to provide extremely strong assembly frameworks, those frameworks being subjected to very large loads in the reactor during operation.
In the case of transient phases during which the flow rate of the cooling water is liable to vary rapidly over time, the fuel assembly can further perform oscillations and the bottom end-piece thereof, when the fuel assembly is lowered under the action of the thrust of the leaf springs, may strike the lower core plate, which may bring about wear and damage when fuel assemblies are used in a nuclear reactor for a long time.
It is therefore necessary to limit as much as possible the maintenance loads applied to the fuel assemblies in order to compensate for the hydraulic thrust of the cooling water which passes through the fuel assembly.
FR-A-2,479,535 proposes a device for limiting the effects of the axial hydraulic thrust on the fuel assemblies of a nuclear reactor so as to be able to dispense with the metal supporting springs at the upper portion of the fuel assemblies and to limit the rapid displacements and the resultant mechanical shocks of the fuel assemblies during transient phases. To that end, there are fixed to the end-pieces of the fuel assemblies, tubular housings that project relative to the end-piece in the axial direction and which are closed at an axial end fixed to the end-piece and which are open at the other end thereof in order to receive a centering pin that engages, in a tight or practically tight manner, inside the housing. The housing further comprises a calibrated hole in the region of the end-piece so that, during the axial displacements of a fuel assembly in the nuclear reactor during operation, those displacements are braked and attenuated by water flowing in the calibrated hole. It is further possible to provide, inside the housing, a split bush comprising flexible arms whose inner diameter is slightly smaller than the diameter of the centering finger.
Such a device is complex and requires a substantial modification of the end-pieces of the fuel assembly. In particular, it is very difficult to modify existing fuel assemblies in order to limit the compression effects applied to the structure of the fuel assembly in order to compensate for the hydraulic thrust on the fuel assembly during operation.